Time difference synchronisation for determination of a property of an optical device

ABSTRACT

The present invention relates to determination of a property of an optical device under test, e.g. the group-delay of the optical device under test, by: a) tuning an optical frequency λ of an optical beam ( 4 ), b) deriving a dependency of the optical frequency λ of the optical beam ( 4 ) over a first time period t, c) deriving a dependency of the optical property of the device under test ( 18 ) over a second time period t+Δt, d) synchronizing the time dependency of the optical frequency λ of the optical beam ( 4 ) with the time dependency of the optical property of the device under test ( 18 ), and e) deriving the frequency dependency of the optical property of the device under test ( 18 ) from the synchronized time dependencies.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to determination of a property ofan optical device, e.g. the group-delay of the optical device. In modernoptical communication systems, the duration of information-carryingoptical pulses is becoming increasingly short. In 40 Gb/s communicationsystems, the data pulses are shorter than 25 ps. Therefore, it isbecoming increasingly important to measure e.g. the group-delay ofoptical devices with an accuracy of better than 1 ps. E.g. a heterodyneoptical network analyzer has the potential to make such extremelyprecise and accurate measurements of a property of an optical device.

SUMMARY OF THE INVENTION

[0002] Therefore, it is an object of the invention to provide improveddetermination of a property of an optical device, in particular toimproved determination of the group-delay of an optical device.

[0003] The object is solved by the independent claims.

[0004] Heterodyne optical network analyzers are for example used formeasurements of group delay in optical components. Typically inheterodyne optical network analyzers or analyzer systems, twointerferometers are involved. A tunable laser source (TLS) launcheslight into the two interferometers, and this light is continuously tunedfrom a start-frequency to a stop-frequency. One of the interferometers,the device-under-test (DUT) interferometer, measures the group delay ofa DUT as a function of frequency. In order to measure this group delay,precise knowledge of the frequency tuning as a function of time isnecessary. This necessary information about the time-dependence of thefrequency-tuning is supplied from measurements made with the secondinterferometer, the reference interferometer. However, two practicallyunavoidable characteristics of such systems interact to create asignificant limitation on the measurement precision of these devices.These two problems are nonlinear sweep of the tunable laser source andlength mismatches present in the system.

[0005] For example, a problem can arise if an extra length of fiber ispresent before the reference interferometer and not the DUTinterferometer. In this situation, an error occurs in measuring thegroup delay of the DUT because the group delay measurement relies onfrequency-tuning rates measured with the reference interferometer. Therates measured by the reference interferometer are, because of the extrafiber length, delayed in time with respect to the actualfrequency-tuning rates appropriate in the DUT interferometer. The sameissue arises when the electronic delays of the photo receivers thatmeasure the optical heterodyne signals are not identical. This situationis equivalent to a longer path leading to or from one of theinterferometers than the other. In the present application the above iscalled an external time-delay.

[0006] A second problem can arise if the two interferometers are notsymmetrical, i.e. if the free spectral range of the interferometers isdifferent. The free-spectral range is inversely proportional to thedifference in length between the two arms of an interferometer. Inalmost all heterodyne optical network analyzers, the two arms of aninterferometer have different lengths. If the difference is not the samein both interferometers, the time-dependence of the frequency-tuningmeasured by the reference interferometer will not correctly describe thetime-dependence of the frequency-tuning used to measure the group delayin the DUT interferometer. This type of length mismatch is unavoidableif one wishes to use the same optical setup to measure several differentDUT with differing lengths. In the present application the above iscalled an internal time-delay.

[0007] With other words: The reason for the time delay can be internal,i.e. the reason for the delay lies within the measurement device, e.g.within one of the arms of an interferometer, used to measure the opticalproperty, and can be external, i.e. the reason for the delay lies notwithin the measurement device, e.g. within one of the arms of aninterferometer, used to measure the optical property, but occurs on theway of the light before entering the measurement device or after havingleft to measurement device.

[0008] These length mismatches become particularly detrimental when theTLS does not tune its frequency linearly. Nonlinear frequency tuningcauses significant errors in the measurement when length mismatches arepresent.

[0009] The present invention proposes a time-delay, applied in hardwareor software, to correct for the length mismatches, electronictime-delays, and nonlinear frequency tuning that ordinarily limit theaccuracy of the measurement of the optical property. An advantage of thepresent invention is therefore improved determination of the group-delayof an optical device by applying a time-delay shift to compensate forinternal and/or external time-delays to compensate e.g. for group delayerrors induced by the interaction of internal and/or externaltime-delays caused by internal and/or external length mismatches,external time-delays caused by electronic time-delays and localoscillator nonlinear frequency sweep.

[0010] The time-delay can be derived theoretically from the setup of theanalyzer system, e.g. mathematically, or can be derived in an empiricway, e.g. by testing several time-delays which are supposed to suit toan used analyzer system and by sorting out the time-delay giving thebest results for this system.

[0011] The time-delay can be a constant value or at least besufficiently approximated thereby. However, the time delay may also bedependent e.g. on the wavelength or might vary overtime. In such casethe time-delay can be varied dynamically or adaptively to the system.However, in case the dependency of the time-delay (e.g. on time orwavelength) is sufficiently small, a static time-delay might besufficient to compensate for the inherent time-delay of the system.

[0012] The time-delay can be introduced in hardware preferably byapplying an electronic delay of the appropriate amount in the receiversystem electronics of a heterodyne optical network analyzer.Alternatively, an extra length of fiber could preferably be applied tothe path leading to one of the detectors. In software, a delay canpreferably be applied numerically to the measured signals.

[0013] In conclusion, according to the present invention a time-delay,applied in hardware or software, can correct errors due to variouslength and electronic delay mismatches in the system and the interactionof these mismatches with nonlinear tuning of the TLS and therefore canultimately enable the full measurement capabilities of a heterodyneoptical network analyzer.

[0014] In the inventive apparatus the beam splitters can be couplers,also. The numbering of the beam splitters, i.e. first, second, third . .. beam splitter, does not necessarily imply that the splitters have tobe different. Actually all or some of them can be the same, e.g. whenused in an interferometer setup of Michelson type.

[0015] Other preferred embodiments are shown by the dependent claims.

[0016] It is clear that the invention can be partly embodied orsupported by one or more suitable software programs, which can be storedon or otherwise provided by any kind of data carrier, and which might beexecuted in or by any suitable data processing unit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Other objects and many of the attendant advantages of the presentinvention will be readily appreciated and become better understood byreference to the following detailed description when considered inconnection with the accompanied drawing. The components in the drawingare not necessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present invention.

[0018]FIG. 1 shows a schematic illustration of a heterodyne opticalnetwork analyzer according to a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] Referring now in greater detail to the drawings, FIG. 1 shows aschematic illustration of a heterodyne optical network analyzer 1according to a preferred embodiment of the present invention.

[0020] The light source for the analyzer 1 is a TLS 2. Light 4 from theTLS 2, as shown in FIG. 1, propagates in an optical fiber 6 toward acoupler 8, where it is split into two paths 10 and 12. The lighttransmitted along path 12 enters a reference interferometer 14, whilelight transmitted along the other path 10 enters a DUT interferometer16, i e. the interferometer comprising the DUT 18. Light propagatingtoward the reference interferometer 14 passes through an additionallength l_(d) in path 12, that is not present in path 10 leading to theDUT interferometer 16. This represents an “external” length mismatchl_(d) in the analyzer 1. While this external delay is depicted as anextra length of optical fiber, an external delay can also occur whenelectronic group delays of two photo receiver systems detecting thesuperimposed signals are not the same. After the light 4 propagates intoeither interferometer 14, 16, it is split by couplers 20 and 22 into twointerfering arms 24, 25 and 26, 27, respectively. The lengths of thesearms 24, 25 and 26, 27 differ, and that difference is represented inFIG. 1 by l₁ in the DUT interferometer 16 and l₂ in the referenceinterferometer 14. A difference between the two lengths l₁ and l₂represents an “internal” length mismatch. The light in these arms 24, 25and 26, 27 ultimately is reflected by mirrors 34, 35 in the referenceinterferometer 14 and by mirror 36 and the DUT 18 in the DUTinterferometer 16 and is recombined at the couplers 20 and 22,respectively. The recombined light 44 and 46 is measured by thedetectors 54 and 56, respectively. Because arms 25 and 27 of eachinterferometer 14, 16 are longer than the other arms 24 and 26,respectively, the light in the longer arms 25, 27 is delayed relative tothe light in the shorter arms 24, 26 as the beams recombine in thecouplers 20 and 22. The time-delay of each interferometer 14, 16 isgiven by $\begin{matrix}{{\tau_{1,2} = \frac{2{nl}_{1,2}}{c}},} & \text{(0.1)}\end{matrix}$

[0021] where n is the index of refraction of the optical fiber along thepaths 6, 10, 12, 24, 25, 26, 27 and c is the speed of light in vacuum.Since the two arms in Eq. (0.1) are assumed to be dispersion less, theindex of refraction is not a function of optical frequency. Likewise,the length mismatch l₂, and therefore, τ₂, are constants; they areassumed to be independent of frequency as is appropriate for reflectionsfrom mirrors. However, the length mismatch, l₁, is a function offrequency because its value depends on the dispersive properties of theDUT 18. The frequency dependence of l₁(ω) also results in a frequencydependence of τ₁(ω).

[0022] To avoid measurement errors when τ₁≠τ₂, the TLS 2 in theheterodyne optical network analyzer 1 should generate light 4 with acontinuous and linear frequency sweep. Current tunable lasertechnologies, however, do not permit perfect linear tuning of thelaser's frequency sweep.

[0023] The radian optical frequency generated by a typical TLS 2 iswritten

τ(t)=2π[v ₀+γ_(t)+χ(t)],  (0.2)

[0024] where v₀ is the optical frequency at the beginning of the sweep,γ is the linear sweep rate, t is time, and χ^((t)) represents thenonlinear component of the sweep. With this definition and theassumption that the intensity, I, of the light output by the TLS 2 isconstant, the intensity measured by the detector 56 in the DUTinterferometer 16 can be written

I(t+τ _(m))=2I+2I cos(Φ_(D)(t+τ _(m))),  (0.3)

[0025] where τ_(m) represents the time it takes for light to travel fromthe TLS 2 to the mirror 36 in the DUT interferometer 16 and back to thedetector 56. The phase of the beat signal can be written as$\begin{matrix}\begin{matrix}{{\phi_{D}\left( {t + \tau_{m}} \right)} = {2{\pi \left\lbrack {v_{0} + {\gamma \quad t} - {\frac{\gamma}{2}\tau_{1}} + {\chi \left( {t - \frac{\tau_{1}}{2}} \right)}} \right\rbrack}\tau_{1}}} \\{= {{{\omega \left( {t - \frac{\tau_{1}}{2}} \right)}\tau_{1}} = {\omega_{1}{\tau_{1}.}}}}\end{matrix} & \text{(0.4)}\end{matrix}$

[0026] Here, the variable Ω₁ has been used as a shorthand notation forthe frequency ψ(t−τ₁/2).

[0027] Also, an assumption was made that the nonlinear component of thefrequency sweep, χ^((t)), varies slowly on the time scales of the orderτ₁. In most situations, this assumption is valid since τ₁ is typicallyon the order of 10⁻⁸ s. The phase of the beat signal measured in thereference interferometer 14 can likewise be derived, and it is found tobe $\begin{matrix}\begin{matrix}{{\phi_{R}\left( {t + \tau_{m} + \tau_{d}} \right)} = {2{\pi \left\lbrack {v_{0} + {\gamma \quad t} - {\frac{\gamma}{2}\tau_{2}} + {\chi \left( {t - \frac{\tau_{2}}{2}} \right)}} \right\rbrack}\tau_{2}}} \\{= {{\omega \left( {t - \frac{\tau_{2}}{2}} \right)}{\tau_{2}.}}}\end{matrix} & \text{(0.5)}\end{matrix}$

[0028] The time-delay τ_(d) is included in Eq. (0.5) to account for the“external” delay incurred by the additional length l_(d) of fiber alongthe path 12 leading to the reference interferometer 14. Experimentally,however, the phase of the reference interferometer 14 is actuallymeasured at the same times as the phase of the DUT interferometer 16.Consequently, it is helpful to make a change of variables such that Eq.(0.5) becomes

Φ_(R)(t+τ _(m))=ω(t−τ ₂/2−τ_(d))τ₂=ω₂τ₂  (0.6)

[0029] Here, as above for ω₁, the variable ω₂ denotes the radian opticalfrequency emitted from the TLS 2 at a time t−τ₂/₂−τ_(d).

[0030] One of the principle measurements of the shown heterodyne networkanalyzer 1 is the measurement of the group delay of the DUT 18. However,other optical properties of the DUT 18 can be measured, also. The groupdelay of DUT 18 over the range of frequencies swept by the TLS 2 can beobtained from the evolution of Φ_(R) and Φ_(D). The group delay isactually defined to be $\begin{matrix}{{\tau_{g}(\omega)} = {\tau_{1} + {\omega {\frac{\tau_{1}}{\omega}.}}}} & \text{(0.7)}\end{matrix}$

[0031] Clearly then, $\begin{matrix}{{\tau_{g}\left( \omega_{1} \right)} = {\frac{\phi_{D}}{\omega_{1}}.}} & \text{(0.8)}\end{matrix}$

[0032] Because Φ_(D) is experimentally measured as a function of timerather than frequency, it is difficult to evaluate Eq. (0.8) directlyusing the measured data. The evolution of Φ₁ is inextricable from theevolution of the phase Φ_(D) since τ₁ varies with optical frequency. Theevolution of the reference interferometer, however, enables us to knowthe evolution of Φ₂, which is simply equal to Φ_(R)/τ₂ because τ₂ is aconstant. This knowledge of the evolution of ω₂ ultimately enables aprecise evaluation of Eq. (0.8). It is important to note, however, that$\begin{matrix}{{\tau_{g}\left( \omega_{2} \right)} \neq {\frac{\phi_{D}}{\omega_{2}}.}} & \text{(0.9)}\end{matrix}$

[0033] To illustrate this more fully, we rewrite Φ_(D) in terms of Φ₂,such that $\begin{matrix}{{\phi_{D}\left( {t + \tau_{m}} \right)} = {\quad{\left\lbrack {\omega_{2} + \quad {2{\pi \left( {{\frac{\gamma}{2}\left( {{2\tau_{d}} + \tau_{2} - \tau_{1}} \right)} + {\chi \left( {t - \frac{\tau_{2}}{2} - \tau_{d}} \right)} - {\chi \left( {t - \frac{\tau_{1}}{2}} \right)}} \right)}}} \right\rbrack \tau_{1}}}} & \text{(0.10)}\end{matrix}$

[0034] Performing the differentiation of Φ_(D) with respect to Φ₂ leadsto $\begin{matrix}\begin{matrix}{\frac{\phi_{D}}{\omega_{2}} = {\tau_{1} + {\omega_{2}\frac{\tau_{1}}{\omega_{2}}} + {2{\pi \left( {\tau_{d} + \frac{\tau_{2}}{2} - \frac{\tau_{1}}{2}} \right)}\left( \frac{^{2}{\chi (t)}}{t^{2}} \right)\left( \frac{t}{\omega_{2}} \right)\tau_{1}}}} \\{{+ 2}{\pi \left( {\tau_{d} + \frac{\tau_{2}}{2} - \tau_{1}} \right)}\left( {\gamma + \frac{{\chi (t)}}{t}} \right){\left( \frac{\tau_{1}}{\omega_{2}} \right).}}\end{matrix} & \text{(0.11)}\end{matrix}$

[0035] Clearly, this is not the group delay τ_(g)(ω₂). The last twoterms are error terms. Under typical experimental parameters, the secondof these terms is negligible. However, the first term can indeed inducesignificant errors.

[0036] These errors can be eliminated, however, by applying a time-delayto the data to compensate simultaneously for “internal” and “external”delays. The proper time-delay is equivalent to a change of variables inEq. (0.6), where$\left. t\rightarrow{t + {\frac{1}{2}\left( {\tau_{2} - \tau_{1}} \right)} + {\tau_{d}.}} \right.$

[0037] Thus, ω₂→ω₁, and the data gained with the detector 54 ofreference interferometer 14 can be used to evaluate Eq. (0.8) correctly.

1. A method of determination of an optical property of an optical deviceunder test (18), comprising the steps of: splitting an incoming lightbeam (4) into a first initial light beam (10) and a second initial lightbeam (12), splitting first initial light beam (10) into a first lightbeam (27) and a second light beam (26), coupling the first light beam(27) into the optical device under test (18), letting the second lightbeam (26) travel a different path as the first light beam (27),superimposing the first (27) and the second light beam (26) to produceinterference between the first light beam (27) and the second light beam(26) in a resulting first superimposed light beam (46), detecting thepower of the first superimposed light beam (46) for deriving a firstsignal over time containing information about the optical property ofthe device under test (18) when tuning the frequency of the incominglight beam (4) over a given frequency range, splitting the secondinitial light beam (12) in a fifth light beam (24) and a sixth lightbeam (25), superimposing the fifth (24) and the sixth light beam (25)after each light beam (24, 25) has traveled a different path, to produceinterference between the fifth (24) and the sixth light beam (25) in aresulting second superimposed light beam (44), detecting the power ofthe resulting second superimposed light beam (44) for deriving a secondsignal over time containing information about the time dependence of thefrequency when tuning the frequency of the incoming light over a givenfrequency range, compensating a time-delay between the first and thesecond signal, and deriving a frequency dependency of the first signalfor deriving the optical property of the optical device under test (18).2. The method of claim 1, further comprising the steps of: derivingelements of the Jones matrix for the optical device under test (18) fromthe compensated frequency dependence of the detected powers.
 3. Themethod of the claims 1 or 2, further comprising at least one of thefollowing steps: using a first light beam (27) with definedpolarization, detecting the power of the resulting first superimposedlight beam (46) as a function of frequency and polarization, andderiving the polarization mode dispersion of the device under test (18)from the information obtained through the measurement, preferablyrepresented as Jones matrix elements of the device under test (18),deriving the chromatic dispersion of the device under test (18) from theJones matrix elements of the device under test (18), using a first lightbeam (27) with defined polarization, detecting the power of theresulting first superimposed light beam (46) as a function of frequencyand polarization, and deriving the principal states of polarization ofthe device under test (18) from the Jones matrix elements of the deviceunder test (18), using a first light beam (27) with definedpolarization, detecting the power of the resulting first superimposedlight beam (46) as a function of frequency and polarization, andderiving the polarization dependent loss of the device under test (18)from the Jones matrix elements of the device under test (18), using afirst light beam (27) with defined polarization, detecting the power ofthe resulting first superimposed light beam (46) as a function offrequency and polarization, and deriving the fast and slow group delays,associated with the fast and slow principal states of polarization ofthe device under test (18) from the Jones matrix elements of the deviceunder test (18), deriving the insertion loss of the device under test(18) from the Jones matrix elements of the device under test (18),deriving the transmissivity of reflectivity of the device under test (2)from the Jones matrix elements of the device under test (2), and/orusing a first light beam (27) with defined polarization, detecting thepower of the resulting first superimposed light beam (46) as a functionof optical frequency and polarization, and deriving higher-orderpolarization mode dispersion parameters, such as the rate of change ofthe differential group delay with frequency, from the Jones matrixelements of the device under test (2).
 4. The method of any one of theclaims 1-3, further comprising the steps of: choosing the time-delay tobe ${{\frac{1}{2}\left( {\tau_{2} - \tau_{1}} \right)} + \tau_{d}},$

τ₂ being the delay of the sixth light beam (25) relative to the fifthlight beam (24), τ₁ being the delay of the first light beam (27)relative to the second light beam (26), (τ₂−τ₁) being an internal delay,τ_(d) being an external delay.
 5. A software program or product,preferably stored on a data carrier, for executing the method of one ofthe claims 1 to 4 when run on a data processing system such as acomputer.
 6. An apparatus for determination of a property of an opticaldevice under test (18), preferably a heterodyne optical network analyzer(1), comprising: a first beam splitter (8) in the path (6) of theincoming light beam (4) for splitting the incoming light beam (4) into afirst initial light beam (10) traveling a first initial path and asecond initial light beam (12) traveling a second initial path, a secondbeam splitter (22) in the path of the first initial light beam (10) forsplitting the first initial light beam (10) into a first light beam (27)traveling a first path and a second light beam (26) traveling a secondpath, wherein the optical device under test (18) can be coupled in saidfirst path for coupling in the first light beam (27), a third beamsplitter (22) in said first and in said second path for superimposingthe first (27) and the second light beam (26) after the second lightbeam (26) has traveled a different path as the first light beam (27) toproduce interference between the first light beam (27) and the secondlight beam (26) in a resulting first superimposed light beam (46)traveling a first resulting path, a first power detector (56) in saidfirst resulting path for detecting the power of the resulting firstsuperimposed light beam (46) traveling the first resulting path as afunction of frequency when tuning the frequency of the incoming lightbeam (4) over a given frequency range, a fourth beam splitter (20) insaid second initial path for splitting the second initial light beam(12) in a fifth light beam (24) traveling a fifth path and a sixth lightbeam (25) traveling a sixth path, a fifth beam splitter (20) in saidfifth and said sixth path for superimposing the fifth (24) and the sixthlight beam (25) after each light beam (24, 25) has traveled a differentpath, to produce interference between the fifth (24) and the sixth lightbeam (25) in a resulting second superimposed light beam (44) traveling asecond resulting path, a second power detector (54) in said secondresulting path for detecting the power of the resulting secondsuperimposed light beam (44) as a function of frequency when tuning thefrequency of the incoming light beam (4) over a given frequency range,whereby an output of the power detector (54) is connected with anevaluation unit for: detecting a time dependence in a tuning gradient ofthe frequency when tuning the frequency of the incoming light beam (4)over the given frequency range, using a time-delay for compensating anexternal and/or an internal time-delay, and deriving the opticalproperty of the optical device under test (18) from the compensatedoptical frequency dependencies of the detected powers.
 7. A method ofdetermination of an optical property of an optical device under test(18), comprising the steps of: a) tuning an optical frequency λ of anoptical beam (4), b) deriving a dependency of the optical frequency λ ofthe optical beam (4) over a first time period t, c) deriving adependency of the optical property of the device under test (18) over asecond time period t+At, d) synchronizing the time dependency of theoptical frequency λ of the optical beam (4) with the time dependency ofthe optical property of the device under test (18), and e) deriving thefrequency dependency of the optical property of the device under test(18) from the synchronized time dependencies.
 8. The method of claim 7,wherein steps b) and c) are performed with the use of at least oneinterferometer.
 9. The method of claims 7 or 8, wherein step d) isperformed by using a time-delay to synchronize the time dependency ofthe optical frequency λ of the optical beam (4) with the time dependencyof the optical property of the device under test (18).
 10. The method ofany one of the claims 7-9, wherein the synchronization is dynamic orstatic.
 11. A method of determination of an optical property of anoptical device under test (18), comprising the steps of: tuning afrequency of an incoming light beam (4) over a given frequency range,splitting the incoming light beam (4) into a first initial light beam(10) and a second initial light beam (12), splitting first initial lightbeam (10) into a first light beam (27) and a second light beam (26),coupling the first light beam (27) into the optical device under test(18), letting the second light beam (26) travel a different path as thefirst light beam (27), superimposing the first (27) and the second lightbeam (26) to produce interference between the first light beam (27) andthe second light beam (26) in a resulting first superimposed light beam(46), detecting the power of the first superimposed light beam (46) forderiving a first signal over time containing information about theoptical property of the device under test (18), splitting the secondinitial light beam (12) in a fifth light beam (24) and a sixth lightbeam (25), superimposing the fifth (24) and the sixth light beam (25)after each light beam (24, 25) has traveled a different path, to produceinterference between the fifth (24) and the sixth light beam (25) in aresulting second superimposed light beam (44), detecting the power ofthe resulting second superimposed light beam (44) for deriving a secondsignal over time containing information about the time dependence of thefrequency, compensating a time-delay between the first and the secondsignal, and deriving a frequency dependency of the first signal forderiving the optical property of the optical device under test (18).